1. Field of the Invention
The present invention relates to a baseband analog circuit suitable as a receiver. More specifically, the present invention relates to a baseband analog circuit suitable as a receiver in, for example, a CDMA (code division multiple access) type mobile phone.
2. Description of the Related Art
A conventional baseband analog circuit 21 will be described with reference to FIG. 2. The baseband analog circuit 21 includes a demodulator 22 for demodulating an input signal into a baseband analog signal, and a low-pass filter 23 having the demodulated signal input thereto. The demodulator 22 is implemented with, for example, bipolar transistors and the output impedance thereof is high. The low-pass filter 23 is also implemented with bipolar transistors.
A transmission signal of frequency 800 MHz is received by an antenna 24 and is subsequently output to a frequency converter 25. At the frequency converter 25, the transmission signal is frequency-converted into an intermediate frequency signal having a frequency of approximately 85 MHz and is output to the demodulator 22. At the demodulator 22, the intermediate frequency signal is demodulated into a baseband analog signal and is output to the low-pass filter 23. Since the baseband analog signal demodulated at the demodulator 22 has a band up to approximately 600 kHz, the cutoff frequency of the low-pass filter 23 is approximately 600 kHz. The baseband analog signal passed through the low-pass filter 23 is output to an A/D converter (analog-to-digital converter) 26, where it is converted into a digital signal.
However, a problem exists in that the low-pass filter 23 produces noise, which decreases the C/N (carrier to noise ratio) because the input impedance of the low-pass filter 23 and the output impedance of the demodulator 22 are large. Another problem is the decrease in the N/F (noise figure) of the low-pass filter 23 because the output impedance of the demodulator 22 is larger than the equivalent noise resistance of the low-pass filter 23.
Accordingly, an object of the present invention is to solve the foregoing problems by providing a low-pass filter with improved noise characteristics.
To this end, according to a first aspect of the invention, a baseband analog circuit is provided. The baseband analog circuit includes a demodulator to demodulate a received signal and to output a baseband analog signal, a low-pass filter to filter the demodulated baseband analog signal, and an impedance converter, provided between the demodulator and the low-pass filter, having a lower output impedance than the output impedance of the demodulator.
The impedance converter may comprise an emitter-follower circuit. The output impedance of the impedance converter, and thus impedance seen by the input terminal of the low-pass filter, is decreased from the conventional receiver. Accordingly, the noise occurring at the low-pass filter is reduced thereby improving the C/N. The output impedance of the emitter-follower circuit may be not greater than the equivalent noise resistance of the low-pass filter. The low-pass filter may comprise a complementary metal-oxide semiconductor (CMOS) circuitry.
In this manner, an inexpensive baseband analog circuit can be provided.